The present invention relates to a method of controlling a work machine equipped with an electronically controlled load-sensing hydraulic system and a control apparatus used for such a method.
FIG. 6 shows an example of conventional electronically controlled load-sensing hydraulic systems installed in a work machine, such as a hydraulic shovel.
Referring to FIG. 6, numeral 1 denotes a motor mounted on a work machine. A variable-capacity type hydraulic pump 2 adapted to be driven by the motor 1 is equipped with a slanted plate control mechanism 2a for controlling the pump output rate. The respective inlet ports of control valves 3,4, which are adapted to change the direction of the hydraulic oil fed by the hydraulic pump 2, are connected to the discharge opening of the hydraulic pump 2. Hydraulic cylinders 7,8, each of which functions as a hydraulic actuator, are respectively connected to the outlet ports of the control valves 3,4. The hydraulic cylinder 7 is connected to the control valve 3 via pressure compensation valves 5a,5b adapted to maintain a constant differential pressure between the inlet and the outlet of the control valve 3, while the hydraulic cylinder 8 is connected to the control valve 4 via pressure compensation valves 6a,6b adapted to maintain a constant differential pressure between the inlet and the outlet of the control valve 4.
An unload valve 9 for releasing the hydraulic oil in the hydraulic pump 2 when the control valves 3,4 are at the neutral position is connected to a pipe line extending from the hydraulic pump 2 to the inlet ports of the control valves 3,4.
Each control valve 3,4 has a port that is located at the center of the control valve as viewed in the drawing. An inlet of a shuttle valve 10 adapted to select the higher pressure between the load pressures respectively introduced from the two control valves 3,4 is connected via pipe lines L1,L2 to the aforementioned ports of the control valves 3,4. When the control valves 3,4 are at the neutral position, the inlet of the shuttle valve 10 communicates with a tank 11.
A pipe line L3 links the outlet of the shuttle valve 10 with respective pilot operation units of the aforementioned slanted plate control mechanism 2a of the hydraulic pump 2, the pressure compensation valves 5a,5b,6a,6b and the unload valve 9.
The slanted plate control mechanism 2a is provided with a control valve 2a1 and an actuator 2a2. The control valve 2a1 is adapted to control the flow of the hydraulic oil so as to maintain the balance between the discharge pressure of the hydraulic pump 2 and the sum of the higher load pressured selected by the aforementioned shuttle valve 10 (hereinafter called xe2x80x98the load-sensing pressurexe2x80x99) and the pressure set by means of a spring. The actuator 2a2 is adapted to be operated by the pressure oil fed through the control valve 2a1 so as to control the angle of inclination of the slanted plate of the hydraulic pump 2.
Electric joy sticks 12a,12b serving as an operating device to be operated by the operator are connected to the input end of a controller 13, which is adapted to perform control and computation based on signals from the joy sticks 12a,12b. The output end of the controller 13 is connected to electro-hydraulic transducing valves 3a,3b,4a,4b attached to the control valves 3,4.
The control valves 3,4 are designed to be operated by the aforementioned electrohydraulic transducing valves 3a,3b,4a,4b so as to control the direction and the flow rate of the hydraulic oil fed from the hydraulic pump 2 to the hydraulic cylinders 7,8 of the work machine.
The electro-hydraulic transducing valves 3a,3b of one of the two control valves, i.e. the control valve 3, is adapted to be controlled by the electric joy stick 12a, while the electro-hydraulic transducing valves 4a,4b of the other control valve, i.e. the control valve 4, is adapted to be controlled by the electric joy stick 12b. 
FIG. 7 is a control block diagram of a conventional controller 13. In FIG. 7, the aforementioned electric joy sticks 12a,12b are connected to function generating units 14a,14b,15a,15b that are adapted to set command signals to electro-hydraulic transducing valves 3a,3b,4a,4b based on electric manipulation signals, which are electric signals input from the electric joy sticks 12a,12b and represent degree of manipulation of the electric joy sticks 12a,12b. Via drivers 16a,16b,17a,17b that are adapted to drive the electro-hydraulic transducing valves 3a,3b,4a,4b, the function generating units 14a,14b,15a,15b are respectively connected to the solenoid portions of the electro-hydraulic transducing valves 3a,3b,4a,4b. 
The electronically controlled load-sensing hydraulic system described above has a configuration such that operating the electric joy sticks 12a,12b causes command signals to the electro-hydraulic transducing valves 3a,3b,4a,4b to be set by the function generating units 14a,14b,15a,15b in the controller 13 so that the drivers 16a,16b,17a,17b drive the electro-hydraulic transducing valves 3a,3b,4a,4b, thereby driving the control valves 3,4.
When the pressure oil has been fed to the hydraulic cylinders 7,8 as a result of the operation of the control valves 3,4, load pressures are respectively applied from the control valves 3,4 through the pipe lines L1,L2 to the shuttle valve 10, by which the higher load pressure is selected to act as the load-sensing pressure and directed through the pipe line L3 to the slanted plate control mechanism 2a of the hydraulic pump 2, the pressure compensation valves 5a,5b,6a,6b and the unload valve 9.
As the load-sensing pressure selected by the shuttle valve 10 is directed to the control valve 2a1 of the slanted plate control mechanism 2a of the hydraulic pump 2, the discharge pressure of the hydraulic pump 2 is set such that it is higher by a predetermined reference pressure than the load-sensing pressure.
As the pressure compensation valves 5a,5b,6a,6b maintain a constant differential pressure between the inlet and the outlet of the control valve 3 and a constant differential pressure between the inlet and the outlet of the control valve 4, pump flow rate that are respectively in proportion to the aperture areas of the control valves 3,4 are distributed to the hydraulic cylinders 7,8.
The conventional electronically controlled load-sensing hydraulic system described above presents a problem in that differences among individual elements actually used as the drivers 16a,16b,17a,17b, electro-hydraulic transducing valves 3a,3b,4a,4b or control valves 3,4 produce a variance in signals from the electric joy sticks 12a,12b when the hydraulic cylinders 7,8 start to operate.
In order to solve the above problem, an object of the present invention is to provide a method of controlling a work machine, wherein said method is capable of preventing differences among the individual elements of the control system that is adapted to control the hydraulic actuators based on electric manipulation signals from producing a variance in said electric manipulation signals at the actuation of the hydraulic cylinders. Another object of the invention is to provide a control apparatus used for said control method.
A method of controlling a work machine according to the present invention relates to a method of controlling a work machine by inputting electric manipulation signals into function generating units and controlling control valves of a hydraulic circuit that is adapted to drive hydraulic actuators of the work machine based on command values output from said function generating units, wherein said control methods includes a process comprised of computing calibration deviations, each of which is computed based on the difference between the command value corresponding to an imaginary start-up moment of a hydraulic actuator, said command value being a provisional value on a reference function that has been set beforehand in the corresponding function generating unit, and the actual command value stored at the moment when said hydraulic actuator was actually started; setting corrected functions in the respective function generating units by adding the calibration deviations computed as above to the respective reference functions; and controlling said control valves based on the command values that have been changed by using the corrected functions.
As described above, the control method of the invention calls for computing calibration deviations based on the differences between the command values corresponding to imaginary start-up moments of the respective hydraulic actuators, said command values being provisional values on reference functions that have respectively been set beforehand in the function generating units, and the actual command values stored when said hydraulic actuators were actually started; setting corrected functions in the respective function generating units by adding the calibration deviations to the respective reference functions; and controlling said control valves based on the command values that have been changed by using the corrected functions. By thus using corrected functions in the function generating units, the control method of the invention eliminates the variance produced in electric manipulation signals as a result of differences among the individual elements actually used in the control system for controlling the hydraulic actuators of the work machine based on said electric manipulation signals, in other words differences among the individual control valves disposed between the function generating units, the hydraulic actuators or the like. The method of the invention thus ensures uniform operation.
According to another feature of the invention, the method of controlling a work machine calls for detecting load-sensing pressure at the load side and control valve return pressure generated in a return circuit, which is located closer to the tank than are the control valves; computing a differential pressure between the load-sensing pressure and the control valve return pressure; and computing a calibration deviation based on the difference between the provisional command value on the reference function, said provisional command value corresponding to an imaginary start-up moment of the hydraulic actuator associated therewith, and the command value stored at the moment when the aforementioned differential pressure rose, which moment is regarded as the actual start-up moment of the hydraulic actuator.
By using the moment when the differential pressure between the load-sensing pressure and the control valve return pressure rises, reliable recognition of the actual start-up moment of a hydraulic actuator is ensured.
According to yet another feature of the invention, the method of controlling a work machine calls for regarding the moment when the differential pressure between the load-sensing pressure and the control valve return pressure exceeds a given reference pressure which is constant and slightly greater than zero as the moment when the differential pressure rises.
As a constant reference pressure slightly greater than zero is used as the standard of determination, the invention is capable of accurately judging the state of rising of the differential pressure at the moment when the differential pressure exceeds the reference pressure and storing the command value at that moment. Thus, the invention is capable of computing an accurate calibration deviation.
A work machine control apparatus according to the invention includes a hydraulic pump, an operating unit adapted to output electric manipulation signals, a controller adapted to perform control computation based on electric manipulation signals sent from the operating unit, electro-hydraulic transducing valves connected to the output end of the controller, control valves adapted to be driven by the electro-hydraulic transducing valves so as to control the hydraulic oil fed from the hydraulic pump to hydraulic actuators of the work machine, and a detecting means for detecting the actual start-up moment of the hydraulic actuators, wherein the controller is provided with function generating units, drivers and calibration computing units such that each function generating unit has a reference function that incorporates the relationship between an electric manipulation signal and a command value to the corresponding electro-hydraulic transducing valve, each driver is adapted to drive the corresponding electro-hydraulic transducing valve based on the output from the corresponding function generating unit, and that each calibration computing unit is adapted to correct the corresponding reference function. Each calibration computing unit is adapted to store the command value sent to the corresponding electro-hydraulic transducing valve at the moment when the corresponding hydraulic actuator was actually started, i.e. the actual start-up moment detected by the aforementioned detecting means; compute a calibration deviation based on the difference between the stored command value and the provisional command value corresponding to an imaginary start-up moment of the hydraulic actuator associated therewith, said provisional command value set beforehand by using the reference function in the corresponding reference function generating unit; and set the corrected function in the reference function generating unit by adding said calibration deviation to the reference function.
As described, above each calibration computing unit of the controller of the work machine control apparatus according to the invention stores the command value sent to the corresponding electro-hydraulic transducing valve at the moment when the corresponding hydraulic actuator was actually started; computes a calibration deviation based on the difference between the stored command value and the provisional command value that corresponds to the start-up moment of the corresponding hydraulic actuator and has been set beforehand by using the reference function in the corresponding reference function generating unit; and sets the corrected function in the reference function generating unit by adding said calibration deviation to the reference function. Therefore, by means of the corrected functions in the reference function generating units, the control apparatus according to the invention is capable of eliminating the variance that has been produced in electric manipulation signals sent from the operating units at the start-up of the hydraulic actuators as a result of the differences among the individual elements actually used in the control system for controlling the hydraulic actuators of the work machine based on said electric manipulation signals, in other words the differences among the individual drivers, the electro-hydraulic transducing valves or the control valves, which are disposed between the function generating units and the hydraulic actuators. Thus, the control apparatus of the invention ensures uniform operation.
According to yet another feature of the invention, the detecting means for detecting the start-up moments of the hydraulic actuators of the work machine control apparatus includes a first pressure detector and a second pressure detector, wherein the first pressure detector is adapted to detect control valve return pressure generated in the return circuit from the control valves, while the second pressure detector is adapted to detect load-sensing pressure at the load side; and each calibration computing unit is adapted to compute the differential pressure between a load-sensing pressure and a control valve return pressure of the corresponding control valve and regard the moment when the differential pressure exceeds a given, constant reference pressure that has been set slightly greater than zero as the actual start-up moment of the corresponding hydraulic actuator.
When a hydraulic actuator is at a standstill, the differential pressure between the load-sensing pressure and the control valve return pressure respectively detected by the two pressure detectors is zero. Therefore, the current state of the hydraulic actuator can be reliably determined by means of the aforementioned reference pressure, which is set slightly greater than zero.